What are the mechanical losses in the motor?

In this article, we will take a closer look at the mechanical losses in motors, which mainly include friction loss, viscosity loss and wind loss, three types.

Friction loss is an important component of mechanical loss. When an object moves, it will generate a force in the opposite direction on the contact surface with other objects. This force will resist the movement of the object, which we call friction loss. Specifically, friction loss can be further divided into static friction loss and dynamic friction loss.

Static friction loss describes that when an object first starts to move, a large force must be applied to overcome the static friction of the contact surface in order to start moving.

Dynamic friction loss refers to the fact that when an object continues to move, there is still a fixed friction force on the contact surface.

Let's understand friction loss more clearly through the following figure. When the object is just started, there will be a large static friction loss; when the object's speed increases to the maximum speed where static friction exists, only a small dynamic friction loss remains.

Friction loss diagram

Translating these concepts to our daily lives, we all have similar experiences. For example, when you try to push an object, you will find that you need to apply a relatively large force to make the object move at the beginning. But interestingly, once the object starts to move, the force required will be relatively reduced, and the object will move smoothly and continuously. This is the actual effect of static friction and dynamic friction.

As for the mathematical expression of friction, we can describe it with the following equation:

Friction formula

Ff is friction, Fc is dynamic friction, Fs is static friction, ν is speed, and νs is the maximum speed at which static friction exists. Through such understanding and mathematical models, we can more accurately grasp the important role of friction loss in motor design and take corresponding measures to minimize these losses and improve the efficiency and life of the motor.

Friction torque reveals

In motor operation, we often use torque to express its efficiency. Therefore, friction loss can be interpreted as follows:

Friction torque formula

Tf represents the friction torque.

Tc represents the dynamic friction torque.

Ts represents the static friction torque.

ω represents the angular velocity.

ωs represents the highest angular velocity at which static friction torque occurs.

Deep rotation friction

Although friction is theoretically affected by speed, in reality the effect is small and occurs mostly at the beginning of movement. Therefore, the friction loss of the motor is often considered to be a fixed value that is independent of speed. This also explains why small motors have lower efficiency.

For example, the bearing friction loss of a motor is 10W. For a 20W motor, this accounts for 50% of the loss, but for a 200W motor, it only accounts for 5% of the loss.

The importance of bearings

Bearing selection is particularly important for small motors. One way to reduce friction loss is to reduce the contact area. Common bearings use spherical balls, the purpose of which is to reduce the contact point and thus reduce friction.

More advanced bearing designs are non-contact, such as magnetic levitation or air suspension principles, which avoid friction losses but still have viscous losses.

Key points:

Unless the motor is really small, most people will choose to ignore friction losses.